Traditionally, a dual SIM mobile phone is one which holds two SIM cards. Initially, dual-SIM adapters were made available to use in regular mobile phones to allow them to contain two SIMs, and to switch from one to the other as required. This combination is called a standby dual-SIM phone.
More recently, some phones have been devised and manufactured to natively work with two separate SIMs, both of which may be active one at a time or at the same time. These are active dual-SIM phones.
Dual-SIM operation allows the use of two different services without the need to carry two phones at the same time. For example, the same mobile phone can be used for business and private purpose with separate numbers and bills. And, in some cases of travel overseas, an additional/different SIM card may be required for the country(ies) or regions visited. Moreover, using multiple SIM cards allows the user to take advantages of the different pricing plans for calls and text messages to certain destinations as well as mobile data usage.
Until recently, the mobile phones have been largely eschewed by larger phone manufacturers due to their close ties with the mobile phone networks who would prefer that customers use only one dedicated network exclusively for technical or financial reasons. However, more recently manufacturers such as Nokia and other competitors have started producing these mobile phones, which will target customers mostly from developing countries.
Several types of adapters are available when the mobile phone is using different SIM cards. When they only allow one SIM to be active, and to receive calls, at any given time; they provide various ways of switching from one SIM to the other. Earlier versions required the phone to be switched off and on again to change from one to another. Later models allow the user to switch SIM cards via a menu or by keying in a numeric code, without switching the phone on and off.